I don't know why this is a hard rule to observe:
Don't follow strangers into holes in the ground!!!
Origin: Iceland and other parts of Scandinavia
Description: Illes are dark, ugly and hairy trolls. They only come out at night. Did I mention they are completely naked?
Disposition: Illes are able to change appearances at will. They play beautiful music and turn themselves into beautiful maidens with the aim of luring the unsuspecting into their underground world. The mortal will pine away for the beautiful maiden after their real form is revealed. One should refrain from touching them as well for their skin will make a person sick.